I. Field of the Invention
This invention relates generally to microwave devices and in particular to wide band ferrite circulators suitable for use at millimeter wave frequencies. Such circulators have many applications including their use as isolators, switches, and as components in reflection-type amplifiers.
II. Description of the Prior Art
Microwave signals with wave lengths shorter than about 1 cm are conventionally referred to as millimeter waves. High performance ferrite circulators intended for use at frequencies below 30 GHz are well known in the prior art. The present invention is generally concerned with ferrite circulators intended for operation between about 40 GHz and 300 GHz and particularly with ferrite circulators intended for operation within the frequency range extending from 75 to 110 GHz, which is commonly referred to as the W-band.
The geometry and dimensions of one or more ferrite elements is the principal factor which determines circulator performance. The way in which the ferrite element is embedded in a waveguide Y-junction is a secondary performance consideration. Ferrite elements dimensioned for fundamental mode operation in the W-band exhibit bandwidths typically from about 1 to 2%.
In an effort to achieve broader bandwidth, ferrite elements have been configured and dimensioned to function at higher order modes. More particularly, a pair of closely-spaced higher order resonant mode sets are used to enable the ferrite element to exhibit the familiar double-ripple response over the broader bandwidth. A detailed discussion of technical factors concerning the design of millimeter wave circulators may be found in U.S. Air Force Technical Report No. AFWAL-TR-81-1199 entitled: "Millimeter Wave Device Development", the disclosure of which is incorporated by reference herein. U.S. Pat. No. 4,638,267 to Holpp provides another example of the well known use of a pair closely-spaced higher order resonant mode sets to enlarge the bandwidth of a ferrite circulator.
When broad band circulators are designed for higher order mode operation above 40 GHz, the suppression of spurious mode resonances becomes a significant problem. Spurious modes cause spike-type disturbances of the return loss and insertion loss functions. In amplifier applications, spurious modes cause irregular gain functions and potential instabilities.
Most W-band circulator designs known in the prior art present significant manufacturing problems because the various junction components must be manufactured to close tolerances and then assembled with precise alignment. The complexity of these prior art junctions typically requires each junction to be individually tested and tuned after assembly. This requirement of individual testing precludes construction of circulators which employ multiple junctions in a single housing.